Filamentary display devices

ABSTRACT

In the mounting of filamentary coils between support terminals, the coils are stretched an amount in the range of 30-120 percent of the original length thereof. After mounting, but prior to the final sealing-in of the devices, the coils are heated to a temperature well in excess of the normal coil operating temperature. The coils are then restretched. After the sealing-in of the devices, the coils are again heated to a temperature well in excess of the normal coil operating temperature.

United States Patent Bonnette [451 July 4, 1972 [54] FILAMENTARY DISPLAY DEVICES [72] Inventor: Richard Arthur Providence, NJ.

[73] Assignee: RCA Corporation [22] Filed: May 26, 1970 [21} Appl. No.: 40,659

Bonnette, New

[52] US. Cl ..29/25.l6, 29/25.15, 29/2517 [51] Int. Cl. ..H01j 9/18, H0lj 9/36 [58] FieldofSearch ..29/25.1,25.11,25.13,25.14,

[5 6] References Cited UNITED STATES PATENTS 2,812,508 11/1957 Mullan ..324l20 X 2,991,387 7/1961 McCauley ..313/109.5 3,408,523 10/1968 Demarest et a1 ..3l3/109.5

3,483,609 12/1969 Stone et a1. ..29/25.16 X 3,529,335 9/1970 Demarest et a1... ..29l25.16 3,564,325 2/1971 Bonnctte et al ..3l3/222 Primary ls'xuminer.lohn F. Campbell Assistant Examiner-Richard Bernard Lazarus Atlumey(1lenn H. Bruestle [57] ABSTRACT In the mounting of filamentary coils between support terminals, the coils are stretched an amount in the range of 30-120 percent of the original length thereof. After mounting, but prior to the final sealing-in of the devices, the coils are heated to a temperature well in excess of the normal coil operating temperature. The coils are then restretched. After the sealing-in of the devices, the coils are again heated to a temperature well in excess of the normal coil operating temperature.

8 Claims, 2 Drawing Figures FILAMENTARY DISPLAY DEVICES BACKGROUND OF THE INVENTION This invention relates to filamentary type display devices, and particularly to the processing and mounting of the filaments of such devices.

One type of filamentary display device comprises an array of electrical resistance filaments individually mounted between pairs of support terminals. By passing an electrical current through various selected ones of the filaments, to heat the filaments to incandescence, various images, e.g., numerals, are displayed.

In order to avoid excessive cooling of the filaments, in order to minimize the amount of power needed to energize the filaments, the only support provided for the filaments is atthe ends thereof, i.e., the filaments are suspended between the 'support terminals. To minimize distortion of the filaments DESCRIPTION OF THE DRAWING FIG. 1 is a front view of a sub-assembly used in filamentary devices of the type in which the instant invention has utility; and

FIG. 2 is a section along line 2-2 of HG. l.

DESCRlPTlON OF PREFERRED EMBODIMENTS Filamentary display devices of the type with which this invention has utility are known. An example of such devices is described in U.S. Pat. No. 3,416,020, to J. A.'Carley, issued Dec. 10, 1968.

In such display devices, as shown in FIG. lherein, a plurali- 4 ty of filamentary coils 10 are mounted between pairs of support and terminal posts 12 and 14, the posts being mounted on a substrate 16. In the illustrative embodiment, two of the posts 14 are each common to two different filaments 10.

Means, not shown, are provided for connecting the posts 12 and 14 to terminal leads of the device, whereby electrical voltages can be applied between individual pairs of posts 12 and 14 to heat the filaments mounted between the post pairs to incandescence. Thus, various images, e.g., the numerals through 9, in the illustrative embodiment, can be displayed.

Each filamentary coil is fonned of a given length of a refractory wire, e.g., tungsten, by which is meant either substantially pure tungsten, or tungsten with such additives as rhenium or thorium oxide. When heated to incandescence, the length of the wire increases. To help prevent distortion of the heated coils, the coils are preferably mounted under tension. According to one known technique, the coils are fabricated with a length less than the distance between the posts between which the coils are to be mounted, and the coils are stretched and tensioned in the mounting operation.

After the various coils 10 are mounted on the substrate 16, the substrate is mounted on a stem, and the stem-substrate assembly is enclosed within a glass envelope, evacuated, and sealed-off. These latter operations are well known, hence not described herein.

In spite of the tension mounting of the coils 10 between the support posts 12 and 14, a problem of the prior art devices is that substantial distortion of the filaments often develops during use of the devices.

The filaments distort in two different modes, namely, as sags and hooks". With. reference to FIG. I, a sag", identified by the reference numeral 20, is an out-of-line bulge or curvature of thecoil 10. In prior art devices, using, for example, coils having an outer diameter of 2 mils and a length of 300 mils, sags in the order of mils often occurred.

A hook, identified by the reference numeral22 in FIG. 1, is an abrupt bend in theaxial directionofthe coil, the hook generally occurring close to a support post 12 or 14. In the priorart devices, hooks in the order of l0 mils often occurred.

Hooks, it presently appears, arise as a result of the process used to bond the coil ends to the'support posts 12 and 14, for example, welding or staking. ln the bonding process, the end turns of the coils which are bonded to the posts become twisted and flattened out of the normal planes thereof, thereby tending to distort the coils. Surprisingly, the hooks do not become noticeable until after the device has been completed and operated for at least a short period of time, e.g., 5 to 10 minutes.

In accordance with the instant invention, substantial reduction in the size of both hooks and sags is achieved, as follows.

It is found that the amount of stretching during coil mountingis important with respect to the elimination of hooks, but is of surprisingly little significance with respect'to the elimination of sags.

lnitially, in the early development of display devices of the type herein described, bythe applicant and his co-workers, the coils .were stretched about'l 2 percent during the mounting of the coils on the posts 12 and 14. Quite by accident, it was discovered that substantial reduction in the size and occurrence of hooks is achieved by stretching the coils during mounting an amount substantially greater than 12 percent, and in the range between 30 percent and 120 percent of the initial length of the coils.

It is found that there is no particular critical amount of stretching required since, to a certain-extent, the incidence of hooks, and'the size thereof, are dependent upon the-degree of care taken in the bonding of the coils to the posts. The greater the care taken and the less distortionof the coils during the bonding operation, the less stretching is required. However, even with'the best quality bonds, stretchings of at least 30 percent appear to be desirable. Stretchings above an upper limit, in the order of 120 percent, however, appear not to further improve the end results. A preferred stretching is in the order of percent.

While the above-described stretching procedure substantially reduces the size of the hooks, further improvement is obtained by the use of what is herein termed a post-flashing" technique. This technique refersto subjecting the various coils to a relatively high voltage heating step after completion of the devices, i.e., after seal-off thereof, but prior to any use of the device at nonnal rated voltages.

The actual temperature to which the coils are heated is not known to a high degree of accuracy owing to the difficulty of measuring high temperature of small objects (e.g., the diameter of the coil wire typically used is less than I mil).'ln general, however, the coils are post-flashed to a temperature well in excess of the usual operating temperature of the coils, which, in one embodiment, is in the order of l,250 C. E.g., the coils are heated to a temperature in the range between 1,900C. and 2,500 C. An optical pyrometer is used to measure the coil temperature.

The particular temperature to which the coils are heated is not critical. ln tenns of applied voltage, a voltage in the order of 2 k to 3 times the rated voltage is preferred.

The duration of the post-flashing operation is also not critical, durations in the range between 2 and 30 seconds'being generally satisfactory. The higher the voltage used, the shorter the time required.

Excessively high voltages,'hence excessively high temperatures, e.g., in excess of 2,500 C., are to be avoided since such high temperatures cause very rapid and excessive embrittlement of the coil wire. Accordingly, the post-flashing operation is preferably performed at thelowest voltage and duration consistent with obtaining satisfactory results.

For the purpose of reducing coil sagging, which otherwise develops during operation of the devices, the coils 10 are subjected to a pre-heating operationprior to the final sealing-in of the device. Again, the coils are heated to a temperature well in excessof the usual operating temperature of the coils. While not critical, temperatures in the range between l,700 C. and 2,200 C. appear to provide best results.

While various coil heating means can be used, a preferred means is to pass a current through the coils. To this end, the substrate-coil sub-assembly shown in FIG. 1 is mounted on a stem, not shown, having various leads connected to the various support posts 12 and 14. An example of such a stem mounting arrangement is shown in the aforementioned patent. Current is then passed through the coils using standard socketlng means.

While various pre-heating schedules can be used to obtain the desired end results, various factors affecting the choice of schedule are noted. While not completely understood, it appears that the pre-heating process effects a change in the crystallographic structure of the tungsten wire. Such change, resulting in the elimination of sag of the filament coils during operation of the device, appears to be temperature and time dependent, i.e., lower processing temperatures require longer processing schedules. Thus, for example, for a given coil having a rated operating voltage of 4.5 volts, substantially similar results, with respect to avoiding sag of the filaments during device operation, are obtained by pre-heating the coil, in a non-oxidizing atmosphere, e.g., vacuum, using a voltage of 12 volts for a period of 1 minute, or a voltage of 9 volts for 2 minutes.

For manufacturing cost reasons, the higher-voltage shorterduration process is preferable.

A further consideration is that the particular pre-heating schedule used appears to affect the amount of distortion of the coils during the pre-heating operation. The higher the voltage used, with attendant shorter process durations, the smaller is the amount of filament distortion which occurs during the process.

A limitation on the voltage used, however, is that excessive voltages, giving rise to excessive temperatures, cause rapid and excessive embrittlement of the wire.

In general, it is found that voltages in the order of 2 to 3 times the rated filament voltage, for a period in the order of l to 2 minutes, provide satisfactory results when the coils are heated under the same ambient conditions as they are normally operated, e.g., in vacuum.

Since heating the filament coils in vacuum involves the use of a bell jar, which is time consuming owing to the need for opening the jar and re-exhausting it, a preferred process is to heat the coils in a reducing gas atmosphere, e.g., 90 percent nitrogen, 10 percent hydrogen, an open-ended inverted chamber being suitable for the process. Since the presence of the gas atmosphere cools the coils, higher voltages are required to obtain the desired heating. In one embodiment, for example, with a reducing gas pressure slightly in excess of atmospheric pressure, to provide a steady flow of gases out of the chamber, a voltage of 35 volts, d.c. or a.c., for 30 seconds, is used. Also, it appears that somewhat better results, with respect to sag, are obtained performing the pre-heating process in a reducing gas atmosphere rather than in vacuum.

I The amount of filament distortion occurring during the preheating process is also dependent upon the particular filamentary material used. Commercially available thoriated tungsten wire (1 percent thorium oxide, balance tungsten and trace impurities), for example, is found to sag considerably less during the pre-heating operation than commercially available rhenium-tungsten wire (3 percent rhenium, balance tungsten and trace impurities). A problem with the use of thoriated tungsten wire, however, is that it is somewhat brittle and rather difficult to form, without excessive loss of product, into the small diameter coils normally used in devices'of the type herein described. While rhenium-tungsten' wire is much more satisfactory with respect to the ease of fabrication of the coils, a problem using this wire is that, even using a high voltageshort duration pre-heatingschedule, a relatively large amount of distortion of the coils occurs during the pre-heating process.

To overcome this problem of excessive coil distortion during the pre-heating step, the coils, after the pre-heating step, are restraightened and retensioned by a restretching operation. This is done, in a preferred embodiment, by outwardly bending the posts 12 and 14 between which the coils are mounted, as shown in FIG. 2. In one embodiment, for exam ple, in which the coils have a length of 300 mils, are preheated at 35 volts for 30 seconds in the aforedescribed reducing atmosphere, the coils are restretched an amount in the order of 15 mils. In general, the coils are restretched an amount necessary to eliminate the distortion caused by the preheating step.

After the pre-heating operation, and the restretching operation, if used, and completion of the devices, little sagging of the coils occurs during subsequent operation ofthc devices. In comparison with identical devices in which the pro-heating process is not performed, the amount of sagging which occurs during device operation is reduced, on average, from 20 mils to 3 mils.

The post-flashing operation, previously described in connection with eliminating hooks, is also effective to further reduce the amount of sagging which occurs during operation of the devices. In one series of tests, for example, comparing devices which were subjected to the pre-heating process, but with or without the post-flashing operation, the use of the post-flashing operation reduced the amount of sag which developed during use of the devices, on average, from 3 mils to 2 mils.

By way of specific example, in one embodiment the various coils are made of 0.4 mil diameter rhenium-tungsten wire. Certain ones of the coils are wound at 1,400 turns to the inch, have an outer diameter of 2 mils, and have an unstretched length of mils. When initially mounted, the coils are stretched to a length of 290 mils, thereby having about 800 turns to the inch. The coils are designed to operate at 4.5 volts at 24 milliamperes. The pre-heating is in the aforedescribed reducing atmosphere, at 35 volts, for 30 seconds. The temperature of the coils is in the order of l,850 C. Thecoils are re-stretched 15 mils. The post-flashing operation is at 12.5 volts, for 10 seconds. The temperature of the coils is in the order of 2,l00 C.

For otherwise identical coils made, however, of thoriated tungsten wire, the pre-heating is done at a slightly higher temperature, using a voltage of 40 volts, for about 15 seconds in a reducing atmosphere. Restretching may or may not be required.

With thoriated tungsten wire coils, post-flashing is generally not required.

lclaim:

l. A method of fabricating filamentary devices comprising filamentary coils mounted between support terminals and sealed within an envelope, said coils, in usage of said device, operating at a pre-selected temperature, said method comprising:

mounting said coils between said support terminals,

thereafter heating said coils to a temperature well in excess of said pre-selected temperature, and

thereafter sealing said coils within said envelope.

2. A method as in claim 1 wherein said coils are heated to a temperature in the range between l,700 C. and 2,200 C.

3. A method as in claim 1 wherein said heating step is performed in a reducing atmosphere. 7

4. A method as in claim 1 in which said coils are stretched in excess of 30 percent of the original length thereof during said mounting operation. 1

5. A method as in claim 1 wherein, between said heating and sealing steps, said coils are stretched.

6. A method as in claim 1 wherein, after said sealing step, said coils are again heated to a temperature well in excess of said pre-selected temperature.

7. A method as in claim 6 wherein, the step of again heating said coils is performed by applying a voltage in the range of 2 w to 3 times the voltage applied across the coils during normal operation thereof.

coils being reduced thereby,

thereafter restretching said coils,

thereafter enclosing said coils in an evacuated envelope,

and

thereafter heating said coils to a temperature in the range between l,900 C. to 2,500 C. for a period between 2 seconds and 30 seconds.

t t I t i 

1. A method of fabricating filamentary devices comprising filamentary coils mounted between support terminals and sealed within an envelope, said coils, in usage of said device, operating at a pre-selected temperature, said method comprising: mounting said coils between said support terminals, thereafter heating said coils to a temperature well in excess of said pre-selected temperature, and thereafter sealing said coils within said envelope.
 2. A method as in claim 1 wherein said coils are heated to a temperature in the range between 1,700* C. and 2,200* C.
 3. A method as in claim 1 wherein said heating step is performed in a reducing atmosphere.
 4. A method as in claim 1 in which said coils are stretched in excess of 30 percent of the original length thereof during said mounting operation.
 5. A method as in claim 1 wherein, between said heating and sealing steps, said coils are stretched.
 6. A method as in claim 1 wherein, after said sealing step, said coils are again heated to a temperature well in excess of said pre-selected temperature.
 7. A method as in claim 6 wherein, the step of again heating said coils is performed by applying a voltage in the range of 2 1/2 to 3 times the voltage applied across the coils during normal operation thereof.
 8. A method of fabricating filamentary devices comprising: providing a plurality of filamentary coils formed from tungsten wire, said coils being of pre-selected lengths, stretching said coils in the range of 30 percent to 120 percent of said pre-selected lengths and mounting said stretched coils between pairs of support terminals, heating said coils in a reducing gas atmosphere to a temperature in the range between 1,700* C. and 2,200* C. for a period between 1/2 and 2 minutes, the tension of said coils being reduced thereby, thereafter restretching said coils, thereafter enclosing said coils in an evacuated envelope, and thereafter heating said coils to a temperature in the range between 1,900* C. to 2,500* C. for a period between 2 seconds and 30 seconds. 